Seismic Activity at Preston New Road: FAQs

Are the earthquakes related to hydraulic fracturing?

Hydraulic fracturing operations started at Preston New Road, near Blackpool, on 15 October. The first event BGS detected in the area was on 18 October. This is not unexpected since hydraulic fracturing is generally accompanied by microseismicity (very small earthquakes that are too small to be felt).

How big are the events?

The largest event that we have detected to date had a magnitude of 1.1 ML. Earthquakes with magnitude less than 2 are not usually felt and if they are felt then by only a few people very close to the earthquake. Earthquakes with magnitudes less than 1 are hardly ever felt. The smallest earthquake felt in the UK had a magnitude of 0.5 and was felt by one person.

The magnitude 0.8 ML event at 11:36 UTC on 26 October was around 200 times smaller than the magnitude 2.3 ML event that stopped hydraulic fracturing operations at Preese Hall in 2011. The maximum recorded ground motion was around 0.1 mm/s. This is well below the limits set for quarry operations (6 mm/s). The BGS network of sensors across the UK routinely detects many blasts from quarry operations around the UK with magnitudes of up to around 2 ML every day.

Why did your website show a magnitude of 0.6 ML which then changed to 0.3 ML?

No events have been removed from our web pages. However our web pages are linked to a database that contains information about all the seismic events that we detect and locate. If the database is changed the web pages will reflect this change. In this case, the event at 14:59 UTC on 25 October was still being analysed and the location and magnitude had not been finalised when the web pages were updated. This resulted in incorrect information appearing on the web pages for a short period of time. The correct location and magnitude for this event are now as listed here.

How often do earthquakes of this size occur?

Earthquakes have an exponential frequency-magnitude distribution, so for unit decrease in magnitude there are approximately ten times as many earthquakes. On average there are around 20-30 earthquakes with a magnitude of 2 ML or greater every year somewhere in the UK or immediate offshore area. That means we should expect several hundred earthquakes with a magnitude of 1.0 ML or above and several thousand with a magnitude of 0 ML or above. The number of earthquakes also roughly scales proportional to an area, so a small region of the UK will have fewer events.

Why are BGS able to detect such small earthquakes?

A dense network of temporary seismic sensors around Blackpool allows us to detect much smaller earthquakes than we are typically able to do in other parts of the UK. The BGS permanent network of sensors across the UK is usually able to detect most earthquakes with magnitudes of 2.0 ML or above anywhere in the UK, though this may vary from place to place and also at different times. All of the events detected near Preston New Road have magnitudes that are far below our normal detection threshold.

How can earthquake magnitudes be negative?

Magnitude scales are logarithmic. This means that the amplitude recorded must decrease 10 times for the magnitude to decrease by 1 unit.

At a station close to a magnitude 2 earthquake, the ground moves about 400 micrometres. So a ground movement of 40 micrometres would signify a magnitude 1 earthquake, 4 micrometres a magnitude 0 earthquake and 0.4 micrometres a magnitude -1 earthquake.

The events are very small, much smaller than we would normally be able to detect. Each horizontal line on the real time seismograms shows 30 minutes of data, so the resolution is insufficient to see anything small. The picture below shows the ground motions recorded on nearby stations for the magnitude 0.8 ML event on 26 October at 11:36. AQ04 is about 1 km away and AQ05 is about 3km.

Ground motions recorded on nearby stations for the magnitude 0.8 ML event on 26 October at 11:36 UTC. The vertical scale shows ground velocity

Why does hydraulic fracturing cause seismicity?

The injection of high pressure fluid into intact rock generates cracks. The cracks grow as a result of tiny seismic events at the tips. This process increases the permeability of the rock, allowing trapped hydrocarbons to be extracted. These events are often referred to as induced and the energy for them is limited by the amount of energy in the injection process. This means the magnitudes are generally small.

The injected fluid can also change pore fluid pressures and deformation in the rock mass. Increased fluid pressures can trigger slip on pre-existing faults by reducing the stress that stops the rocks on either side of the fault from sliding. These are often referred to as triggered events and the energy for them comes largely from previously accumulated strain energy. The earthquakes that occurred during hydraulic fracturing operations at Preese Hall, near Blackpool, in 2011 are thought to have been triggered events. The largest of these had a magnitude of 2.3 ML.

What other activities can induce earthquakes?

Activities such as underground mining, impoundment of deep artificial water reservoirs, oil and gas extraction, geothermal power generation, waste water disposal in deep wells and groundwater abstraction have all resulted in cases of induced seismicity. In addition, activities such as blasting during quarry operations can also result ground vibrations that are comparable to small earthquakes. Mining is by far the most common cause of induced seismicity, followed by reservoir impoundment

Tremors in the coalfields of Britain have been reported for at least the last hundred years, and work by the BGS and others led to the conclusion that these events were related to ongoing mining activity and that these were quite distinct from the natural background seismic activity of the UK. In the 1980s and 1990s mining events accounted for approximately 25% of all the earthquakes recorded in the UK. Since the rapid decline of mining activity in the UK there has been a sharp decrease in the number of these events.

Seismicity in the Staffordshire coal fields. Earthquake activity is coloured by time and correlates closely with areas of known mining. Grey shaded areas show the Mining Reporting Areas (Coal Authority data).

What is earthquake magnitude?

It is a measure of earthquake size and is determined from the logarithm of the maximum displacement or amplitude of the earthquake signal as seen on the seismogram, with a correction for the distance between the focus and the seismometer. This is necessary as the closer the seismometer is to the earthquake, the larger the amplitude on the seismogram, irrespective of the size or magnitude of the event. Since the measurement can be made from P, S or surface waves, several different scales exist, all of which are logarithmic because of the large range of earthquake energies (for example a magnitude 6 ML is 30 times larger, in terms of energy than a magnitude 5 ML). The Richter local magnitude (ML) is defined to be used for 'local' earthquakes up to 600 km away, and is the magnitude scale used by BGS when locating UK earthquakes.

Surface wave magnitude (Ms) is based on the maximum amplitude of the surface wave having a period of 20 + 2 s. It is used for observations near the earthquake epicentre where the surface wave is larger than the body wave. This scale applies to any epicentral distance or type of seismograph.

Body wave magnitude (mb) is calculated from the body waves (P,PP,S) and are usually used at larger distance from the earthquake epicentre (P-wave attenuation is less than surface waves, with distance). It can be used for any earthquake of any depth.

Moment magnitude (Mw) is considered the best scale to use for larger earthquakes as the Ms saturates at about magnitude 8. Moment magnitude is measured over the broad range of frequencies present in the earthquake wave spectrum rather than the single frequency sample that the other magnitude scales use.

For comparison purposes, a magnitude 5 ML earthquake is equivalent to the explosion of 1,000 tons of TNT whereas a magnitude 6 ML earthquake is the energy equivalent of 30,000 tons of TNT or a 30 kilotonne nuclear explosion.

Since hydraulic fracturing operations started at Preston New Road, near Blackpool, we have detected some small earthquakes close to the area of operations. Details of these events can be found here. This is not unexpected since hydraulic fracturing is generally accompanied by microseismicity (very small earthquakes that are too small to be felt).